[Fourier Basis [1]}]
For any probability space \((\Omega ,\gamma )\) , there exists an orthonormal basis \(\phi _0,\phi _1,\ldots ,\phi _{|\Omega | - 1}: \Omega \rightarrow \mathbb {R}\)  with \(\phi _0 \equiv 1\)  under which any function \(f \in L^2(\Omega ,\gamma )\)  can be uniquely expressed as
\(f = \sum _{\sigma = 0}^{|\Omega | - 1} \widehat{f}(\sigma )\phi _\sigma .\) 
